1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind, which forcibly changes the air-fuel ratio of a mixture supplied to the engine, to thereby improve exhaust emission characteristics of the engine.
2. Prior Art
There is conventionally known a method of forcibly changing the air-fuel ratio of a mixture supplied to an internal combustion engine (hereinafter referred to as "perturbation of the air-fuel ratio"), in order to improve the purification rate of an exhaust gas-purifying device which purifies exhaust gases emitted from the engine by means of a catalyst. Further, there has been proposed a method of controlling the mode of the perturbation of the air-fuel ratio, for example, by Japanese Laid-Open Patent Publication (Kokai) No. 2-11841, which controls, e.g. the central value of the perturbed air-fuel ratio, the ratio between a time period during which the air-fuel ratio is changed to the rich side (enriching period) and a time period during which the air-fuel ratio is changed to the lean side (leaning period), etc., in response to an output from an oxygen concentration sensor arranged in the exhaust system at a location downstream of the exhaust gas-purifying device. According to the proposed method, the central value of the ratio between the enriching period and the leaning period is set to 1, and a correction amount for correcting a deviation of the actual ratio from the central value is set to a value proportional to the difference between the output from the oxygen concentration sensor and a reference value thereof.
The above proposed method, however, does not contemplate deterioration of the catalyst accommodated in the exhaust gas-purifying device, i.e. a change in the oxygen storage capacity of the catalyst. As a result, in some cases, the air-fuel ratio is forcibly changed to such an extent as exceeds the oxygen storage capacity of the catalyst, which can degrade exhaust emission characteristics of the engine.
Further, according to the proposed method, when the difference between the output from the oxygen concentration sensor and the reference value is small, the correction amount of the above ratio is also small. Therefore, the time period over which the air-fuel ratio is held on the lean side can become long to such an extent as exceeds the storage capacity of the exhaust gas-purifying device, or conversely, the time period over which the air-fuel ratio is held on the rich side can become long with no oxygen stored in the catalyst, which results in degraded exhaust emission characteristics of the engine.